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Full text of "Handbook Of Chemical Engineering - I"

SECTION IX LEACHING AND DISSOLVING
Br J.   V.   N.   DOER1  AND   F.   F.   PETERS2
The term "leaching" as used hereafter is taken to mean the dissolving of a constituent of a comminuted material and the separation of the resultant solution from the undissolved portion or residue. Two general methods are in use: first, percolation, in which dissolution and the separation of the dissolved material are done coincidently, and second* dissolution by agitation of a finely divided pulp held in suspension and separation of the solids by decantation or filtration or both.
In the latter case the dissolving is usually distinct from the separation of the dissolved material and the term "leaching" is often applied to the former. The theories of leaching especially those relating to dissolution will be first discussed, followed by their practical applications.
Theoretical Discussion.—The problem involved is almost identical with tLa* of dissolving any solid in a suitable liquid and is governed by the same physical and chemical laws, if that chemical action which dissolves the solid is the only reaction taking place in the solution at the time. Dissolving salt in water is governed by the same laws which govern the dissolving of calcium carbonate by hydrochloric acid or the leaching or gold and silver from their ores by cyanide (see Ostwald on "Heterogenous Reactions")-
The velocity of dissolution is dependent upon the following factors:
1.  Physical condition or nature of the matter to be dissolved.
2.  Velocity of diffusion.
3.  The concentration of the solvent or in other words the strength of the solution.
4.  The rate of change of relative position between particles of solid and the adjacent liquid.
5.  Temperature.
Effect of Physical Condition.—The increased exposure of surface due to fine grinding will be discussed later but two interesting phenomena should be mentioned here. (1) When the finest portion of a suitable crushed soluble material is added to a saturated solution of the same it will often dissolve to cause super-saturation which effect could never be gotten by contact with the coarser particles. (2) The rate of dissolution for different faces of the same crystal usually varies. These facts point to the necessity of making tests on the actual material to be treated when considering a leaching problem.
In the theoretical discussion that follows it will be assumed that where the soluble material is contained in an insoluble matrix it is so finely pulverized that as dissolution proceeds all particles of the former will be exposed to the action of the solvent and our problem is to consider means of accelerating dissolution.
Effect of Diffusion.—The process of dissolution seems to take place in two steps; ins., first there is a reaction between the solid and the solvent in immediate contact which is followed by the second step, the diffusion of the products of this
1  The Dorr Co., 101 Park Ave., New York City.
2  Westport, Conn.
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